SAS operator Jonathan Williams returns from deployment to his husband, surgeon Daniel Mercer, for a long-overdue reunion in their Kensington flat. Their two weeks of domestic peace—gaming, cooking, slow mornings—shatter when an asteroid strikes London, unleashing a virus that transforms the infected into cannibals within hours.

The government quarantines the city and recruits Daniel to Harrow House, a decommissioned MI5 safehouse fifty miles northwest, where a team of specialists races to find a cure. Jonathan, refusing to let Daniel face danger alone, assigns himself as his husband's personal guard.

They navigate the facility's tense military-medical culture together, stealing moments in shared quarters, communicating in touches disguised as assistance.